Increase buffer size
Increase buffer size

Summary:
When compiling with GCC>=7.0.0, "db/internal_stats.cc" fails to compile as the data being written to the buffer potentially exceeds its size.

This fix simply doubles the size of the buffer, thus accommodating the max possible data size.
Closes https://github.com/facebook/rocksdb/pull/1635

Differential Revision: D4302162

Pulled By: yiwu-arbug

fbshipit-source-id: c76ad59

SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB